Could be the mix, but are you sure it isn't weeds in your area blowing in?

I agree, it's probably more of a windborne issue. I used miracle grow potting soil in all my containers and I have no weeds. I'm on an 8 story balcony, so while I am definitely pulling fluffy white weed spores out of the pots I'm probably not getting the same kind of weed seeds I'd have if I were near the ground. Try mulching your containers to keep the weeds down.

My miracle grow potting soil doesn't soak up water. I planted a bunch of seeds, with some tiny ones laying on top. What can I do, or how would you recommend watering?

I repeatedly find clumps of green sprouts coming up in my miracle grow soil mix after I plant annuals. I'm thinking maybe the chipmunks who are always digging in my pots may be sowing the seeds.